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Astrophysics

Abstract: We have identified perhaps the largest major galaxy merger ever seen. While
analysing Spitzer IRAC images of CL0958+4702, an X-ray selected cluster at
z=0.39, we discovered an unusual plume of stars extending $\gtrsim$110 kpc
outward from the bright central galaxy (BCG). Three galaxies 1-1.5 mag fainter
than the BCG lie within 17 kpc (projected) of the BCG and are probably
participating in the merger. The plume is detected in all four IRAC channels
and at optical wavelengths in images from the WIYN telescope; the surface
brightness is remarkably high ($μ_r\approx$24.8 mag arcsec$^{-2}$ at 50 kpc).
The optical and infrared colors are consistent with those of other BCGs,
suggesting that the plume is composed of old stars and negligible recent star
formation (hence a "dry merger"). The luminosity in the plume is at least
equivalent to a 4L^* galaxy. A diffuse halo extending 110 kpc from the BCG in
one IRAC image suggests the total amount of diffuse light is L_r\sim
1.3x10^{11}h^{-2} L_sun. A Chandra observation shows an X-ray image and
spectrum typical of moderate-mass clusters. We use MMT/Hectospec to measure 905
redshifts in a 1 deg^2 region around the cluster. The velocities of two of the
BCG companions indicate a merger timescale for the companion galaxies of
$\sim$110 Myr and $\sim$0.5-1 Gyr for the plume. We conclude that the BCG and
intracluster light of CL0958 is formed by major mergers at moderate redshifts.
After the major merger is complete, CL0958 will likely become a fossil cluster.